Field of the Invention
The present invention relates to an image forming apparatus employing an electro-photographic system.
Description of the Related Art
As an image forming apparatus employing an electro-photographic system, there has been known a so-called inline-system image forming apparatus that sequentially forms images through a plurality of image forming units (hereinafter, also referred to as “stations”) corresponding to respective colors of yellow (Y), magenta (M), cyan (C), and black (K). A contact development system is widely employed in such an inline-system image forming apparatus. In the contact development system, development is executed in a state in which a development roller serving as a development unit is in contact with a photosensitive drum.
For example, as described in Japanese Patent Application Laid-Open No. 2007-213024, the contact development system image forming apparatus includes a contact-separation mechanism. The contact-separation mechanism brings a photosensitive drum and a development roller into contact with each other when image formation is executed, and separates the photosensitive drum and the development roller from each other when the image formation is not executed. The contact-separation mechanism can switch between three states, i.e., a full-color contact state, a monochrome contact state, and a fully-separated state. In the full-color contact state, development rollers and respective photosensitive drums of stations of all colors are brought into contact with each other. In the monochrome contact state, for example, a development roller and a photosensitive drum of a black station are brought into contact with each other. In the fully-separated state, the development rollers and the respective photosensitive drums of the stations of all the colors are separated from each other.
The three states are switched by the following methods. One method is a method of sequentially switching the following states (1) to (3) (hereinafter, also referred to as “all-state shift type”): (1) Shift from the fully-separated state to the full-color contact state; (2) Shift from the full-color contact state to the monochrome contact state; and (3) Shift from the monochrome contact state to the fully-separated state. In addition, another method is a method of selectively switching the following states (4) and (5) (hereinafter, also referred to as “independent shift type”): (4) Shift from the fully-separated state to the full-color contact state and shift from the full-color contact state to the fully-separated state; and (5) Shift from the fully-separated state to the monochrome contact state and shift from the monochrome contact state to the fully-separated state. In such a contact development-system image forming apparatus, when a full-color image is to be formed, image formation is started after the contact-separation mechanism shifts a state to the full-color state. Further, when a monochrome image is to be formed, image formation is started after the contact-separation mechanism shifts the state to the full-color contact state or the monochrome contact state.
In the contact development-system image forming apparatus, image formation is executed while switching the development rollers and the respective photosensitive drums of the stations of respective colors to the contact state or the separated state. In the above-described system, as described in the conventional technique, image formation may be executed by stations of a part of the colors instead of stations of all the colors. In such a case, there is a problem of image quality degradation. More specifically, switching a station of one color that does not execute image formation between the contact state and the separated state degrades the quality of an image formed by a station of another color.